The present invention relates to the drilling and production from offshore wells and particularly wells in water depths beyond those where conventional divers may operate. As the search for petroleum deposits in offshore waters continues, it has moved into deeper and deeper waters, beyond the depths at which divers can operate. The depths have increased to the point where the use of conventional bottom-supported production platforms is no longer practical. This has led to the use of floating production platforms which require the use of remotely operated subsea wellhead assemblies. The remotely operated equipment includes the installation of various wellhead assemblies during drilling and production such as conventional blowout preventer assemblies as well as lower marine riser assemblies. In addition, after the drilling is completed, other assemblies must be remotely attached to the wellhead, for example, marine risers and production flow lines. All of these assemblies require that they be remotely connected and removed from the subsea well.
The conventional approach to the attachment and removal of various subsea assemblies has relied upon the remote actuation of various latching means. For example, conventional dog-like members have been moved into and out of engagement with the permanently installed subsea well assembly by either mechanical or hydraulic means. For example, hydraulic fluid could be utilized to move a cam which would move the latching members into a latched position or, alternatively, into an unlatched position. Removal of the hydraulic pressure would allow a spring means to move the latches into an unlatched position or, alternatively, into a latched position. Similarly, weight on the tubing string used for running the assembly to the subsea wellhead could be either removed or applied to operate the latching mechanism.
While the various remotely operated latching means have a high degree of reliability, there is always the possibility that the latching mechanism will fail to operate or will become stuck in a latched position. In these circumstances, it would be impossible to remove the subsea assembly. These conditions will normally occur after the assembly has been in place for an extended period of time. Typically, only the production equipment is subject to these conditions since the drilling equipment is normally in place only a short period of time during the actual drilling of the well. Whenever the latching means fails to operate for any reason, some means must be provided for an emergency release of the subsea assembly to allow other operations to be performed.
In U.S. Pat. No. 4,086,776, there is disclosed a release means for releasing a cam-operated locking means used to lock the end of a guide line assembly to a subsea well assembly. The means shown comprise a "scissors" arrangement which is opened to force the locking means into an unlocked position. The scissors arrangement is shown as hydraulically operated by a submersible vessel.
The system disclosed in the '776 patent requires only a modest effort to move the locking means to an unlocked position and does not forcefully remove the guide line assembly from the subsea wellhead assembly. Even if the locking means is moved to an unlocked position, it is still possible that the locking dogs will remain in place and make it impossible to remove the guide line assembly from the subsea assembly.